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The construct of intellectual disability has developed over centuries and has had different functions at different times; from a concept that was used to describe people from whom society needed protecting from in the late to 19th and early 20th centuries, to one used to describe people who are unable to cope in the current environment. It is now defined in terms of having a measured IQ below a fixed cut off point, usually 70, and a low level of adaptive behaviour also often specified in terms of being below a cut off point. Intellectual Disability demonstrates that neither IQ nor adaptive behaviour can be measured with sufficient accuracy for fixed cut off points to be used and suggests a number of new much more loosely defined constructs of intellectual disability based on clinical judgment.

A diagnosis of intellectual disability (ID), mental retardation (MR) or Learning Disabilities (LD) can have a major effect on people’s lives. On the positive side it can provide services, finance, help in schools, and can even prevent the recipient from being executed (Flynn 2006; Flynn 2007; Schalock et al 2007). On the negative side it is may be regarded as a stigmatizing label that an individual may seek to avoid (Baroff 1999). \ud \ud Currently a necessary though not sufficient part of the diagnosis of ID is having an IQ below a specified figure, usually 70, or two standard deviations (SDs) below the norm (American Association on Mental Retardation, 2002; American Psychiatric Association 2000; Department of Health 2001; British Psychological Society 2001). This specification of a specific IQ figure implies that an individual has a “true IQ” that can be measured. \ud \ud If different IQ tests systematically measure either higher or lower than other tests it would raise the question as to which IQ test was providing the best estimate of an individual’s true IQ. In the absence of a test that is clearly an accurate measure of true IQ, the best that could be done would be to decide which of the many IQ tests is likely to be the most accurate, and take that as the “gold standard” assessment against which other assessment should be compared. The Wechsler assessments, the WISC-IV and the WAIS-III should have a good claim to be regarded as the gold standard assessments. However, for these assessments to be accepted as gold standard assessments for the diagnosis of ID, it requires that they should produce equivalent IQ scores in the low ability range. We compared the WISC-IV and WAIS-III on a group of 16-year-olds in special education. Sixteen being the age group at which both assessments can be used.\ud \ud Both the WISC-IV and the WAIS-III were given to seventeen 16-year-olds (9 female and 8 male) attending local special schools in counterbalanced order. The results were as follows: \ud \ud WISC-IV WAIS-III Diff r\ud Mean Full Scale IQ 53.00 64.82 11.82 .93\ud \ud In spite of a very high correlation between the two scales there was a highly significant difference between the mean FS IQ on each scale of nearly 12 points (P

The measurement of Low IQ is subject to far more error than has previously been acknowledged. The five point error that is usually acknowledged only takes into account the internal constancy of the test. When other errors due to changes in assessment conditions, the floor effect, the Flynn Effect and the lack of consistency between test the error is about 18 points.